This invention relates to thermomechanical pulp production, and more specifically to a method of recovering the thermal energy from the pressurized process steam generated during the production of thermomechanical pulp.
Wood pulping techniques based upon chemical or mechanical procedures are well known. Recently a new mechanical wood pulping procedure has been developed. This procedure is based upon the well known fact that long, flexible and undamaged fibers can be produced by softening the lignin with heat. Utilizing the new pulping procedure, a new mechanical pulp quality is produced which is referred to as thermomechanical pulp. One of the benefits of utilizing this new mechanical procedure is that the total pulp output approximately equals the quantity of wood fed into the process.
One of the major drawbacks of thermomechanical pulp production is that a considerable amount of heat is wasted during the process. This heat is in the form of steam which is generated during thermomechanical pulp production. The wasted heat may exceed 80% of the electrical energy used in the process. With an increase in the use of thermomechanical pulp plants, and with the simultaneous increase in the cost and availability of energy resources, there has been an increase in the need to recover the wasted heat and in this way recover the thermoenergy produced during the thermomechanical pulp process.
In a typical thermomechanical pulp plant, there is provided two high pressure refining phases. The refining pressure is approximately 1.5 to 3.5 bars (abs.) with the temperature approximately equal to the temperature of the saturated steam in each of the refining phases.
In the thermomechanical pulp process, wood chips are first scrubbed with water and are fed with a plug screw or a sluice feeder to a pressurized preheater where chips are heated with steam. From the preheater, the chips are then fed by additional screw mechanisms to a first refining stage. After refining, steam and fibre material are separated with a first phase cyclone.
From the first phase cyclone the pulp is fed by means of screws to a secondary refining stage after which the pulp and steam are again separated with a second phase cyclone. The pulp is then directed to a pumping container where it is suitably diluted and from which it is pumped to a sorting mechanism. The pulp is now available as ready made raw material for use in a paper manufacturing machine.
When such a typical thermomechanical pulp process is utilized in the manufacture of news print, the quantity of waste heat generated is approximately 2,000 kWh of energy per one ton of pulp. Of this, approximately 1.2 tons of steam is generated during the primary refining process and approximately 1.0 tons of steam is generated during the secondary refining process.
The steam which leaves during the course of the thermomechanical pulp process does so through threee different outlets. The steam produced during the primary refining process can leave partly from the first phase cyclone and part from the preheater. The steam produced during the secondary refining process can leave partly from the first phase cyclone and partly from the second phase cyclone. The quality of steam is different from the point of view of its air content, inert gas, and mechanical and chemical contaminants contained therein such as the fibres. The temperature of the atmospheric steam is approximately 98.degree. to 99.degree. C.
It should also be noted that a large quantity of the thermal energy created during the thermomechanical pulp productions leaves in the form of "dirty" low pressure process steam.
It is therefore appreciated that a large amount of thermal energy is wasted during the course of the thermomechanical pulp process and such energy leaves in the form of the steam utilized during the process. As a result, there have been attempts to recover such heat in various types of recovery systems where the atmospheric steam generated during this process is utilized to provide heat in various other situations. For example, such heat recovery systems utilize the steam as a remote heating source for housing developments, for heat ventilation air and/or sanitary water, as well as to preheat combustion and/or drying air of a paper manufacturing machine.
Because of heat recovered at other points of the paper manufacturing process, the amount of the heat which is needed in a typical paper mill is much lower than the volume of heat generated in the production of thermomechanical pulp. Since thus far there has been no way to utilize this heat economically, the generated steam is generally blown toward the ambient atmosphere which again increases heat losses and lowers the profitability of the process.
It has been previously suggested that the process steam produced during the thermomechanical pulp process should be utilized for the drying section in the paper manufacturing machine itself. However, when such process steam is directly applied to the drying section, since the process steam contains various contaminants including mechanical and chemical contaminants as well as uncondensed gasses, corrosion will result on the thermal surfaces in the drying sections. At the same time, contaminants will accumulate on these surfaces and make heat transfer more inefficient.
According to a recent suggestion by Torsten Simmons, Varmeatervinning vid T-massaabriken i Braviken, Svensk Papperstidning, No. 10,1977, the thermal energy of the process steam of a thermomechanical pulp plant can be utilized be sending it to a heat exchanger whereby the heat energy is utilized to vaporize clean water. At that point it has been suggested that the pressure of the clean steam produced is then raised by approximately 0.5 bar with a compressor. After this, the high pressurized steam is utilized in the dryer section of the paper machine. However, one of the problems with this suggested procedure is that the use of the compressor and the raising of the vapour pressure adds additionally to the cost of the recovery system and also requires additional energy consumption in order to achieve the recovery objectives.